Direct-reading frequency meter



June 5, 1956 P. G. HANSEL.

DIRECT-READING FREQUENCY METER Filed Dec. 9. 1950 Nami United StatesPatent() DIRECT-READING FREQUENCY METER Paul G. Hansel, Greenvale, N.Y., assigner to Servo Corporation of America, New Hyde Park, N. Y., acorporation of New York Application December 9, 1950, Serial No. 200,060

7 Claims. (Cl. S24-79) My invention relates to a secondary frequencystandard, Ias for frequency-metering applications.

It is an object of the invention to provide an improved device of thecharacter indicated.

It is another object to provide an improved directreading frequencymeter adapted for the measurement of unknown frequencies and for thecomparison of frequencies.

It is also an object to provide an improved referencefrequency generatorto deliver a known frequency as a secondary standard.

Another object is to provide an improved means for the provision oferror signals, representing frequency deviations of a given signal froma reference signal (of known frequency), as for application to thestabilized frequency control of the rst-mentioned signal.

It is a still further object to provide a secondary frequency standardor meter requiring no point-by-point calibration procedures.

It is a` general object to meet the above objects with a frequency metercapable of indicating directly, to any arbitrarily desired resolution,the frequency of an injected signal, the operation being entirelyautomatic and the equipment relatively simple.

Other objects and various further features of the invention will becomeapparent or will occur to those skilled in the art from a reading of thefollowing speciiication in conjunction with the accompanying drawings.In said drawings, which show, for illustrative purposes only, apreferred form of the invention- Fig. l is a schematic layout of afrequency meter incorporating features of the invention;

Fig. 2 graphically depicts a frequency-scanning and counting operationin the device of Fig. 1; and

Fig. 3 schematically illustrates one form of the spectrum generator inFig. l.

Briefly stated, my invention contemplates establishment of a spectrum ofharmonic frequencies to be used as reference frequencies. This spectrummay be based upon a single crystal oscillator so that all harmonics mayreflect the accuracy of the single crystal; the spectrum may includeharmonics of a submultiple of the base frequency of the crystal.Frequency-responsive spectrum-probing means, which may include afree-running search oscillator, may be tuned continuously over afrequency range including a band within the said harmonic spectrum, anda counter may respond to frequency coincidences between the tunedcondition of the probing means and the spectrum of harmonic frequencies,so as to count the number of such coincidences for a particular tuningsearch or sweep probling means. In use as a frequency meter, that is,when it is desired to measure an unknown frequency, the probing meansmay be caused panoramically to scan a given part of the harmonicspectrum, with the counter responding to the number of harmonicstraversed by the probing sweep. When the oscillator reaches the unknownfrequency, the counter responds to the latter coincidence to stopfurther count- Patented June 5, 1956 ing, and the nal count is preservedas a frequency indication.

To simplify the counting procedure, the invention additionally providesa narrow band-pass filter capable of selecting a particular one of theharmonics in the harmonic spectrum, and the coincidence between thetuned probe condition and the filter-selected frequency may be utilizedto derive a reference signal to start operation of the counter. Thus,the counter need only respond to the number of harmonics traversed inthe total sweep, beginning at the filter-selected harmonic and extendingto the harmonic immediately preceding the unknown frequency.

In use as a secondary frequency standard, manual means may be providedin conjunction with the counter to select a particular frequency, andthe tunable probing means may be caused to sweep over the spectrum untilthe counter has attained the count which has been preselected asrepresenting the desired signal frequency. Upon attainment of thiscount, an automatic frequencycontrol circuit may be activated so as tostop further sweep of the probing means and to stabilize the oscillatorat the then-attained particular harmonic frequency. ln use as anerror-signal generator, as for slaving a transmitter oscillator to agiven selected secondary-standard source frequency, thetransmitter-oscillator output frequency may be mixed with the abovementioned preselected secondary-standard source frequency, and knownmeans may respond to the resulting beat frequency to derive an error ordeviation signal.

Referring to the drawings, a harmonic spectrum may be generated by acrystal-controlled harmonic-spectrum generator 1G delivering a spectrumof signals. Each signal may be of a different frequency, but having anintegral harmonic relationship to the basic crystal frequency Fn. Asuitable technique to provide such a spectrum is illustratedschematically in Fig. 3; according to this technique, and depending uponthe desired resolution of the device, a particular submultiple of thenatural crystal frequency F0 is selected, and the harmonies of theresulting submultiple frequency fo are employed. As a means forarbitrarily selecting a base frequency from which the counting functionis to operate, I provide a single-frequency selection filter 11, whichmay be a narrow band-pass filter having a pass band that is preferablyno greater than the frequency spacing between adjacent harmonics in thespectrum of generator l0. The filter 11 may be of a manually tunedvariety, and the setting of the control knob 12 may determine variouslevelsl (as, for example, given multiples of the basic frequency Fo ofthe crystal) of single-frequency selection within the total spectrum ofgenerator 10, depending upon the general region within the spectrum inwhich the unknown frequency fu lies or is thought to lie. The output ofthe filter 11 may thus be a selected reference frequency fr.

In order panoramically to scan or sweep the spectrum of generator 10, Iemploy frequency-responsive spectrumprobing means which may include atuned circuit with means for sweeping the tuned frequency thereofthroughout the harmonic spectrum and with detector means responsive tofrequency coincidences in the harmonic spectrum; however, in the presentform, I provide a freerunning oscillator 13, which may employconventional sweep means 14, such as a motor drive on the oscillatortuning capacitor, or a reactance tube controlled by a sweep circuit. Theoutput of oscillator 13 may be combined in a non-linear element or mixer15 to derive a signal upon each sweep of the oscillator-output frequencypast the reference frequency fr selected by the filter l1; such a signalmay be utilized to initiate the counting function, as has beenfunctionally shown by the enabling connection 16 to a counter-controlcircuit 17 for the digital counter 18.v Thecounter 1S. may includeadisplay window 19 within which will appear a given number representingthe solution of the counting process.

Once the counter-control circuit hasbeenV enabled by a start-countimpulse delivered in line 16, the counter-control circuit 17 may respondto all register impulses delivered in line 20. It will be understoodthat such register impulses` may be derived in a manner analogous toderivation of the start-count impulse, as by connecting a mixer 21 torespond both to the sweep frequency f5 of the oscillator 13 and to thefull harmonic spectrum (fo nfs) of generator 1t).

Thel unknown or undetermined frequency fu may be appliedY to myfrequency meter at a4 terminal 22 for direct connection to one side of amixer 23, which is also responsive to the sweep frequency fs produced inthc output of oscillator 13. Upon coincidence of the sweep frequency fswith the frequency fu of the unknown signal, the mixer 23 may generate acontrol impulse for stopping the count or for reading the countregistered by the counter 13, as will be understood.

In Fig. 2, I have displayed a portion of the spectrum in which theunknown frequency fu occurs. In the case shown, the unknown frequencyhappens to occur at a point which fails to coincide with one of theharmonic frequencies in the spectrum produced by generator 19. In orderto determine a reference frequency or harmonic near the unknownfrequency fu, I adjust the position of the knob 12 so that a referencefrequency fr occurs in the general neighborhood of the unknownfrequency; in the case shown, the reference frequency selected is 100me. In the normal automatic tuning cycle of the sweep-oscillator means13-14, the entire (or a relatively large part of the) harmonic spectrumwill be swept, but no count will be registered until the counter-controlcircuit 17 is enabled, upon coincidence of the sweep frequency fs withthe reference frequency fr. Thereafter, the counter-control circuit 17may respond to all the register impulses produced upon sweeping theoscillator frequency fs past successive harmonic frequencies in thespectrum of generator 10. In the case shown in the drawings, the spacedfrequency marks in the spectrum will be understood to be purelydiagrammatic, in that too few are depicted on the present scale toprovide any great number of significant figures in the count indicationat window 19; but it will be understood, as suggested above. that theharmonic marks in the spectrum may be as close together as desired,depending upon the desired resolution of the system.

Upon coincidence of the sweep frequency fs with the unknown frequencyfu, an impulse will be generated by mixer 23 to stop further counting.The counter 1S will then display an indication of the difference betweenthe single frequency selected by placement of knob 12 and the mostrecently swept harmonic frequency, that is, the harmonic swept justprior to coincidence with the unknown frequency fu. If desired,interpretation of the counter reading may be facilitated by means of amechanical connection, designated schematically by the dotted line 25,between the knob 12 and the counter-control circuit 17, so that allcounts following an enabling of the counter-control circuit 17 may beadded to a counterdisplay number representing the single referencefrequency fr selected by the filter 11; the display at window 19 maythen be direct-reading.

It will be understood that if it is desired to resolve the unknownfrequency with finer detail, conventional methods of interpolation maybe employed to determine the spectral placement of the unknown frequencybetween adjacent harmonic frequencies in the spectrum of generator 10.

As indicated generally above, my equipment may be further utilized forthe generation of a selected secondary standard frequency, based on theharmonic output of the generator 10. In such applications, a fixed-countselector knob. 26` may be employed inY conjunction with thecounter-control circuit 17 to select a given counting limit at which anenabling signal may be produced in an output line 27. Further, switches28-29 (shown by a. dashed-line interconnection to be mechanicallyganged) may be closed to introduce an automatic-frequency-controlcircuit 30 into controlling relation with the oscillator 13, onceactivated by an enabling signal in line 27. If desired, a sweep-stoppingrelay or other means 31 may also respond to activation of theautomatic-frequencycontrol circuit 30 for the positive stopping offurther sweep tuning, so that the oscillator 13 may be more readilystabilized by the automatic-frequency-control circuit 30.

In operation as a secondary-frequency-standard source, the knob 26 maybe turned until the desired source frequency is read on the counterdisplay 19. The sweep-tuning means may then be activated so as to causesweeping through the spectrum of generator 10. Upon passing thefrequency selected by filter 11, the start-count impulse in line 16 willenable the counter-control circuit, and counting will proceed with eachsuccessive register impulse in line 20 until such time as the countselected by the knob 26 has been reached. Upon reaching the selectedcount, any impulse from circuit 17 will be delivered in line 27 toactivate the automatic-frequency-control circuit 30, which may thenrespond to the particular harmonic frequency which caused such response.Activation of circuit 30 serves to stop further sweep tuning of theoscillator 13 and to slave the oscillator 13 to that particular harmonicfrequency. The output of oscillator 13 may then be available at terminal32 to provide the desired output signal.

As also indicated generally above, my equipment may be utiaized as ameans for stabilizing an external oscillatory circuit which it may bedesired to tune to a selected secondary standard frequency. In suchcase, the output frequency of the external circuit may be applied as theunknown signal fu at terminal 22, and, once the oscillator 13 has beenslaved to the automatc-frequency-control circuit 30, the mixer 23 mayyield an error signal of frequency f@ representing the instantaneousdeviation of the externally generated frequency fu from thesecondarystandard frequency then available as the oscillator frequencyfs. If desired, known means (not shown) may respond to the error-signalfrequency fe to derive an error signal of magnitude proportional tofrequency deviation from the standard frequency'.

It will be appreciated that I have described a relatively simplefrequency-standard control means having direct application to themetering of frequencies and to the generation of known frequencies forexternal comparison. My equipment may be based entirely upon a singlecrystal oscillator, and no point-by-point calibration procedures arerequired, as in present-day frequency meters.

While the invention has been described in considerable detail and apreferred form illustrated, it is to be understood that various changesand modifications may be made within the scope of the invention asdefined in the appended claims.

I claim:

l. In' a frequency-measuring device of the character indicated, aspectrum' generator of harmonics of a reference frequency,frequency-responsive spectrum-probing means tunable over a bandincluding a plurality of harmonies in the output of said generator,frequency-coincidence responsive means connected to said probing meansand to said generator and generating an output signal for eachfrequencycoincidence, counter means connected to said last-defined means andresponsive to frequency coincidences between the output of saidgenerator and of said' probing means to count the number of suchcoincidences` in a given tuning' sweep of said probing means, and secondfrequency-coincidence responsive means connected to said probing meansand including an input connection for application of a source of unknownfrequency, said counter means being also connected to said secondcoincidence-responsive means and being further responsive to a frequencycoincidence between a given unknown frequency and the output of saidprobing means to stop count registration upon such latter coincidence,whereby when said counter has been stopped it may preserve a responseindicative of the particular harmonic frequency swept just beforeattainment of the unknown frequency.

2. In a frequency-measuring device of the character indicated, aspectrum generator of harmonics of a reference frequency, filter meansconnected to said generator and selecting a particular one of saidharmonics, frequency-responsive spectrum-probing means tunable over aband including a plurality of harmonics in the output of said generator,first frequency-coincidence responsive means connected to said probingmeans and to said generator and generating an output count signal foreach frequency coincidence, second frequency-coincidence responsivemeans connected to said probing means and to said filter means andgenerating an output control signal when said probing means sweeps theselected harmonic, third frequency-coincidence responsive meansconnected to said probing means and including an input connection forapplication of a source of unknown frequency and generating an outputcontrol signal when said probing means sweeps the unknown frequency, andcounter means connected to all said coincidence-responsive means andcounting the number of said count signals occurring between one and theother of said control signals.

3. A device according to claim 2, wherein said filter means includes aselectively variable tuning element for tuning to a selected one of saidharmonics for any one of several selectively tuned conditions thereof.

4. A device according to claim 3, in which said counter means includescounter-control means, and means interlocking said tunable element andsaid counter-control means, whereby said counter may be direct-readingwhatever the reference harmonic selected by said selector means.

5. A device according to claim 2, in which said generator includes areference oscillator having a known base frequency, frequency-dividingmeans connected t0 said oscillator for deriving a submultiple of saidbase frequency, and harmonic-generating means connected to said dividingmeans for deriving a spectrum rich in har monics of said submultiplefrequency.

6. In a frequency-measuring device of the character indicated, aspectrum generator of harmonics of a reference frequency, selector meansconnected to said generator and selecting a particular one of saidharmonics, a tunable oscillator and means for sweeping the tunedfrequency thereof over a band of said harmonics including said oneharmonic, frequency-coincidence means connected to said oscillator andto said selector means and to said generator and including an inputconnection for application of a source of unknown frequency, saidlastdetined means generating separate count signals upon coincidence ofthe instantaneous oscillator frequency with harmonics in said band andalso generating separate control signals upon coincidence of theinstantaneous oscillator frequency with said selected one harmonic andwith said unknown frequency, and counter means connected to saidfrequency-coincidence means and responsive t0 said respective controlsignals to count the number of said count signals occurring between saidcontrol signals for a given sweep of said oscillator.

7. A device according to claim 6, in which said counter means includesdigital-display means.

References Cited in the le of this patent UNITED STATES PATENTS2,337,328 Hathaway Dec. 21, 1943 2,490,404 Bliss Dec. 6, 1949 2,490,500Young Dec. 9, 1949 2,510,381 Cushing June 6, 1950 2,523,106 FairbainSept. 19, 1950 2,539,673 Peterson Jan. 30, 1951 2,632,036 Hurvitz Mar.17, 1953

